1. Field of the Invention
This invention relates to an novel process for dewaxing light and heavy oils in two parallel reactors, each containing a different porous crystalline catalyst.
2. Description of the Related Art
It is known to treat gas oil fractions, i.e., petroleum fractions having an initial boiling point of at least about 330.degree. F., so as to selectively remove paraffinic hydrocarbons therefrom. This technique is desirable in order to permit many of these fractions to meet a pour point standard. In particular, many light gas oil fractions, that is, those which are used for No. 2 fuel (home heating oil) and/or Diesel fuel, have pour points which are too high to permit their intended use. A typical pour point specification is 0.degree. F., whereas it is not uncommon for such gas oil fractions to have untreated pour points of 50.degree. F. or higher.
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular composition of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 230.degree. C. (450.degree. F.), the moleuclar weight of the hydrocarbon constituents of the lubricating oils is high and these constituents display almost all conceivable structures and structural types. This complexity and its consequences are fully discussed in "Petroleum Refinery Engineering", by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition), relevant portions of this text being incorporated herein by reference for background.
In general, the basic notion in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties, such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a number of subtractive unit operations which remove the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined, the initial crude oil would be reconstituted.
Unfortunately, crude oils suitable for the manufacture of lubes are becoming less available due to the constant depletion of reserves. In addition, the reliability of a steady, adequate supply from a known source is also a matter of concern.
The desirability of upgrading a crude oil fraction normally considered unsuitable for lubricant manufacture to one from which good yields of lubes can be obtained has long been recognized. The so-called "hydrocracking process", sometimes referred to in the art as "severe hydrotreating", has been proposed to accomplish such upgrading. In this process a suitable fraction of a poor grade crude, such as a California crude is catalytically reacted with hydrogen under pressure. The process is complex in that some of the oil is reduced in molecular weight and made unsuitable for lubes, but concurrently a substantial fraction of the polynuclear aromatics is hydrogenated to form naphthenes and paraffins. Process conditions and choice of catalyst are selected to provide an optimal conversion of the polynuclear aromatic content of the stock, since this component degrades the viscosity index and stability of the stock. Also, in the hydrocracking process, paraffins can be isomerized, imparting good viscosity index characteristics to the final lube product. A hydrocracking process for upgrading crude oil fractions and for dewaxing the hydrocrackate over ZSM-23 zeolite is disclosed in U.S. Pat. No. 4,414,097, the entire contents of which are incorporated herein by reference. Another upgrading process of oil stocks involves solvent refining thereof to extract out undesirable high molecular weight polynuclear aromatic compounds and nitrogen compounds.
Hydrocracked lube stocks and solvent refined stocks, such as, for example, light neutral furfural raffinate, however, tend to be unstable in the presence of air when exposed to sunlight. On such exposure, a sludge is formed, sometimes very rapidly and in substantial amounts. This tendency in a lubricating oil is unacceptable. Additionally, some hydrocracked lube oils tend to darken or to form a haze.
Several methods have been proposed to correct the above-described instability. U.S. Pat. No. 4,031,016 proposes to add certain antioxidants to the hydrocracked oil. A second proposed approach is to hydrotreat a hydrocrackate. Variants of this approach are described in U.S. Pat. No. 3,666,657, which teaches a sulfided mixture of an iron group metal and a Group VI metal for a subsequent hydrotreating stage; in U.S. Pat. No. 3,530,061 which utilizes a hydrotreating catalyst having one or more elements from Group IIB, VIB and VIII of the Periodic Table of Elements at hydrogen pressure up to about 100 psig; and in U.S. Pat. No. 4,162,962 which teaches hydrotreating a hydrocrackate at a temperature in the 200.degree. C. to 300.degree. C. range with a catalyst of prescribed pore size. U.S. Pat. No. 3,530,061 teaches a non-cracking support for a subsequent hydrotreating stage. U.S. Pat. No. 3,852,207 teaches the hydrotreating of oils with a noble metal hydrogenation component supported on an oxide. The patents cited above are believed representative of the state of the art, and each is incorporated herein by reference.
Hydrocracked and solvent refined lubricating oils generally have an unacceptably high pour point and require dewaxing. Solvent dewaxing is a well-known and effective process, but it is expensive. More recently catalytic methods for dewaxing have been proposed. U.S. Pat. No. Re. 28,398, the entire contents of which are incorporated herein by reference, describes a catalytic dewaxing process wherein a particular crystalline zeolite is used. To obtain lubricants and specialty oils with outstanding resistance to oxidation, it is often necessary to hydrotreat the oil after catalytic dewaxing, as illustrated by the teachings of U.S. Pat. No. 4,137,148. U.S. Pat. Nos. 4,283,271 and 4,283,272 teach continuous processes for producing dewaxed lubricating oil base stock including hydrocracking a hydrocarbon feedstock, catalytically dewaxing the hydrocrackate and hydrotreating the dewaxed hydrocrackate. Both of the latter patents teach the use of a catalyst comprising zeolite ZSM-5 or ZSM-11 for the dewaxing phase. U.S. Pat. No. 4,259,174 teaches the dewaxing of a lubricating oil stock having certain characteristics over a catalyst comprising synthetic offretite. U.S. Pats. Nos. 4,222,855 and 4,372,839, the contents of which are incorporated herein by reference, teach catalytic dewaxing processes for waxy hydrocarbon feedstocks over various catalysts exhibiting specified properties, including a catalyst comprising zeolite ZSM-23.
It is inferentially evident from the foregoing background material that the manufacture of modern high quality lubricants in general requires that the crude be treated in a sequence of fairly complex and costly steps. It is further evident that there is a need for processes which can efficiently provide such lubricants from interchangeable and readily available low grade crudes.
It is an object of the present invention to provide an improved process for catalytically dewaxing two different grades of previously-refined oil chargestocks: a relatively light petroleum chargestock and a relatively heavy petroleum chargestock, in a single, integrated process.
It is a further object of the invention to provide a method for manufacturing lubricating oils having a low pour point and good resistance to light.
These and other objects will become apparent to those skilled in the art from the study of the following specification and appended claims.